1. Field
Embodiments herein generally relate to methods for syngas production. More particularly, the embodiments relate to methods for pretreating gasification feedstocks and efficient introduction of feedstock into reactors for syngas production.
2. Description of the Related Art
Gasification is a high-temperature process usually conducted at elevated pressure that converts carbon-containing material into mostly gaseous mixtures, including carbon-dioxide, carbon monoxide, hydrogen and methane. These gaseous mixtures are typically referred to as syngas. Upon production, syngas can be used as a feedstock to generate electricity and/or steam, a source of hydrogen, and for the production of other organic chemicals. Thus, gasification adds value to low-value feedstocks by converting them to marketable products. Coal, crude oil, coke, and high-sulfur residues have been used as gasification feedstock.
The feedstock is treated to minimize water carryover to the gasifier reactor to improve energy efficiency and lower water-shift reactions in the reactor. The feedstock is reacted in a gasifier (i.e. reactor) with an oxidizing medium in a reduced (stoichiometrically oxygen-starved) atmosphere at a high temperature and (usually) high pressure. In the gasifier, a water-gas shift reaction can occur where at least a portion of the carbon monoxide, with catalyst and/or high temperature, is reacted with water to produce predominantly hydrogen and carbon dioxide.
In certain gasifier systems, the feedstock has been fed in the form of a slurry, such as a coal-water mixture or a coal-solvent mixture via pumps to transfer the feedstock from the lockhoppers to the gasifier system. In other systems, the feedstock is typically treated in a lockhopper system that uses one or more flow control valves to distribute the treated feedstock from the lockhopper to the reactor system. Conventional lock hopper systems utilize nitrogen gas to provide an inert atmosphere therein. Nitrogen gas serves as an inert replacement for air where coal oxidation is undesirable.
The presence of nitrogen gas within the conventional lock hopper system, however, creates difficulties when transporting the treated feedstock to the gasifier. The lock hopper system is limiting because of leakage of gas across the rotary valves. Further this feed system does not improve the quality of feedstock to the gasifier, i.e. no increase of volatiles readily available for gasification nor feedstock porosity to increase the reaction rate and distribute the feed uniformly in the reactor.
A need, therefore, exists for methods for pretreating gasification feedstocks to alleviate or remedy such problems.